Use of the fluidized bed combustion chamber has proved to be an advantage in the operation of gas turbines, since by using a pressurized fluidized bed it is possible to drive a gas turbine with many different kinds of fuel, among other things with solid fuels. However, such a combustion chamber, when at operating conditions, represents a substantial store of energy, and this energy store must be taken into consideration if a rapid load drop out on the turbine arises, especially if a solid fuel is being used in the combustion chamber. During such a load drop out, the combustion chamber is by-passed and blocked off from the compressor and the turbine so that the supply of hot combustion gas from the combustion chamber to the turbine ceases. At the same time, the combustion chamber has to be reduced to zero power and possibly also relieved of pressure.
In this connection, there are in principle two different possibilities for tackling the problems posed by the now redundant, but still fully operational combustion chamber.
One possibility involves equalizing the pressure between the cold and hot sides of the combustion chamber, so that the fluidized bed collapses and, since the air supply thereto is stopped, so that the combustion within the bed also practically ceases. Thereafter, in many cases, the bed will be cooled by cooling tubes passing through the bed, through which tubes, water, steam or gas flow. When a long drop out period is anticipated, the pressure in the combustion chamber should, at an early opportunity, be discharged to the surrounding environment in some way.
The other possibility is to allow combustion within the fluidized bed to continue after the fuel supply has been stopped, and to use the compressed air which remains in and around the combustion chamber to burn off the residual fuel that remains in the fluidized bed, the hot gases from the bed now being discharged to the surroundings.
Whichever method is chosed for dissipating the stored energy in the combustion chamber, it is most important not to choose a method which is something in between these two methods, since this could lead to the development of large quantities of carbon monoxide which, together with the compressed air remaining in the container surrounding the combustion chamber, could introduce a grave risk of an explosion occurring.